In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, for future generations of mobile communications systems frequency bands at many different carrier frequencies could be needed. For example, low such frequency bands could be needed to achieve sufficient network coverage for terminal devices and higher frequency bands (e.g. at millimeter wavelengths (mmW), i.e. near and above 30 GHz) could be needed to reach required network capacity. In general terms, at high frequencies the propagation properties of the radio channel are more challenging and beamforming both at the network node at the network side and at the terminal devices at the user side might be required to reach a sufficient link budget.
The terminal devices and/or the transmission and reception point (TRP) of the network node could implement beamforming by means of analog beamforming, digital beamforming, or hybrid beamforming. Each implementation has its advantages and disadvantages. A digital beamforming implementation is the most flexible implementation of the three but also the costliest due to the large number of required radio chains and baseband chains. An analog beamforming implementation is the least flexible but cheaper to manufacture due to a reduced number of radio chains and baseband chains compared to the digital beamforming implementation. A hybrid beamforming implementation is a compromise between the analog and the digital beamforming implementations. As the skilled person understands, depending on cost and performance requirements of different terminal devices, different implementations will be needed.
FIG. 1 illustrates a schematic example of a radio transceiver device 300 with analog beamforming implemented in a terminal device. The radio transceiver device 300 comprises a baseband (BB) processing block 340 operatively connected to an analog antenna array. The antenna array has 6 antenna elements 370 with one gain controller 360 and phase shifter 350 per antenna element 370. The gain controller and phase shifter are used for setting beamforming weights for the antenna elements. As schematically illustrated in FIG. 1 the antenna elements 370 are pointing in different directions in order to achieve omni-directional-like coverage for the radio transceiver device 300.
Terminal devices with analog beamforming are expected in many cases to use reciprocity based uplink (UL) beamforming. That is, the beamforming weights used for uplink transmissions can be based on downlink (DL) measurements. However, in some cases this will not be possible, for example if the analog beamforming is not calibrated between DL and UL, for frequency-division duplexing (FDD) applications, or if different antennas are used for DL and UL at the terminal device. For such cases it could be cumbersome to find the proper beamforming weights for UL transmissions.
A typical closed-loop precoding scheme, such as used in e.g. Long Term Evolution (LTE) telecommunication systems for digital antenna arrays, where one reference signal is transmitted from each antenna element and the TRP evaluates different precoders and feeds back the best precoder index to the terminal device, cannot be applied for analog beamforming implementations. This is because it is not possible to transmit a reference signal for an isolated antenna element. Instead, for analog beamforming the transmission automatically occurs simultaneously on all antenna elements in the antenna array.
Hence, there is a need for an efficient way to acquire beamforming weights of an analog array, especially for UL transmissions.